Titanium metal has been conventionally used for materials and parts for aviation widely; this titanium metal has been developed for various uses, and it is widely used in building materials, road materials, sporting goods, and the like.
Conventionally, titanium metal has been obtained by producing titanium sponge by the Kroll method (titanium tetrachloride is reduced by magnesium metal to yield titanium), crushing the titanium sponge, forming briquettes by pressing the ground titanium sponge, forming a consumable electrode by combining plural briquettes, and melting the electrode of the briquettes by employing VAR furnace, generally.
However, in order to satisfy the recent demand for improving the purity of titanium ingots, in many cases, an electron-beam furnace, in which a titanium sponge is melted by an electron beam and the molten titanium is flowed into a mold to obtain an ingot, is employed instead of the vacuum arc furnace. In particular, among the electron-beam furnaces, an electron-beam hearth melting furnace is frequently used because of a superior purifying ability.
In the case of using an electron-beam furnace, melting titanium and production of a titanium ingot are performed in the furnace under vacuum pressure in the range from 10−3 to 10−4 Torr, which is lower than that in a vacuum arc furnace by 1 to 2 orders of magnitude. Therefore, a high level purification can be realized. As a result, high pure titanium having a purity of 4N to 5N can be produced. However, the electron beam furnace is kept under a high vacuum condition, therefore titanium itself as well as impurities is also evaporated, which is leading a problem in that impurities and titanium are condensed and deposited on the furnace wall and the ceiling wall of the electron-beam furnace, and the impurities and titanium may react with materials of the furnace wall and the ceiling wall, which produce the other compounds.
In this way, the amount of impurities and compounds (hereinafter referred to as “impurities or the like”) produced on the furnace wall or the like, are increased in proportion to the heat number of the melting. In this situation, if the impurities or the like attached on the furnace wall are not controlled, the impurities or the like themselves cannot bear their own weights and they will fall into the lower portion of the furnace. If the impurities or the like fall into the molten metal in the mold or hearth, the impurities are newly incorporated into the mold or hearth and which will lower the ingot quality.
Therefore, in the maintenance operation of the conventional electron beam furnace, the metallic powder and metallic agglomeration adhering to the furnace wall are removed as much as possible after completing a sequence of melting processes and pulling the titanium ingot out of the furnace.
Depending on the capacity of the furnace and the adhering situations on the inner wall, this maintenance operation typically takes about 3 to 7 days until the furnace is ready to be reused. The next melting operation cannot be performed during the maintenance operation, resulting a decrease of the furnace availability. Under recent circumstances in which the titanium metal demand has greatly increased, it is urgently required that the furnace availability is increased by reducing the time for the melting and the maintenance operation as much as possible.
Conventionally, the maintenance operation is conducted by hand; however, for example, a method without operator's contribution has been suggested, in which method the furnace is washed with high pressure water as a solution for decreasing the furnace availability, (see Japanese Unexamined Patent Application Publication No. 2004-183923). Since high pressure water is employed to remove adhered material in this method, it is expected that the maintenance operation can be completed in a relatively shorter time than that of the maintenance operation by hand.
Several improvements have been made in addition to the above-mentioned methods, and in particular, an improving technique has been disclosed, regarding the ceiling wall or the like of an electron-beam furnace. For example, an apparatus for holding a condensed material has been disclosed, in which apparatus a concave water-cooled surface is arranged on the hearth to collect condensed material of evaporated components of an alloy processed in the hearth (see Japanese Unexamined Patent Application Publication No. Hei 11 (1999)-132664). By this technique, since a large number of concave parts, called a plectrum structured parts, are arranged on the ceiling wall of an electron-beam furnace, solid impurities deposited on the concave parts are effectively inhibited to fall downwardly.
Furthermore, a method for preventing the falling of impurity metal evaporated and deposited in the furnace have been disclosed including a process in which Nb—Al alloy is melted by a heating means in a vacuum, and it is held in a hearth arranged in the furnace while Al is evaporated and purified Nb is flowed into a water-cooled crucible, resulting Nb ingot, and a process in which a rotator with a cooling means is arranged above the water-cooled crucible and a woven stainless net is movably attached below the rotator to keep the evaporated Al the adhering and solidifying state on the woven stainless net with near uniformity and density, (see Hei 11(1999)-061288). In these techniques, impurities and titanium vapor evaporated from a melted pool can be condensed on a circular mesh plate at an upper area of the melted pool held in the mold, and as a result, the amount of the condensed material on the ceiling wall of the electron-beam furnace can be reduced.
Furthermore, a technique has been disclosed, in which technique the entire area of the hearth of the electron-beam is covered with a condensation plate arranged directly there above, and impurities evaporated from the hearth are condensed and caught on the plate, resulting an prevention of impurities contamination from the ceiling wall, (see U.S. Pat. No. 5,222,547).
However, although the time required for washing can be shortened to some extent by the method disclosed in Japanese Publication No. 2004-183923, the washing time of the furnace is not reduced because a certain time is required for washing. The melting process must be in waiting during the washing processes, and it is not solved that the furnace availability is not essentially reduced. In addition, the techniques disclosed in Japanese Publications No. Hei 11 (1999)-132664, No. Hei 11 (1999)-061288, and U.S. Pat. No. 5,222,547 are all aimed to trap titanium vapor and impurities evaporated from the melted titanium layer, held in the hearth or mold, before they reach the ceiling wall or at the ceiling wall. Solid material is deposited in each technique; however, there is no guarantee that the solid material will not fall into the molten metal pool, depending on the degree of deposition.
In particular, in the case of a titanium ingot for aviation, the ingot should not contain HDI (High Density Inclusions) or LDI (Low Density Inclusions) extremely and impurities into the ingot is not allowed even in the smallest amounts. For such applications, it is essential for developing an electron beam furnace that can efficiently produce titanium ingots with controlling impurities level precisely under the high rate of the furnace availability.